(Ca,Ce)(Ti,Mn)O₃ Perovskites for Two-Step Solar Thermochemical H₂ Production

Two-step STCH cycles with off-stoichiometric redox-active metal oxides are potentially a cost-effective, sustainable, carbon-neutral pathway to H₂. To overcome the suboptimal solar-to-fuel efficiencies of state-of-the-art CeO₂-based STCH reactors, researchers have begun to explore redox-active ABO₃ perovskites due to their compositional and structural flexibility. While research over the last decade has identified (Sr,La)(Mn,Al)O₃ (SLMA) and Ba(Ce,Mn)O₃ (BCM) perovskites as potential next-generation redox-active metal oxides, they have yet to supplant CeO₂-based materials as the preferred redox-active material for STCH.

In this talk, I will show – using density functional theory (DFT) with a meta-generalized-gradient exchange-correlation functional and Hubbard U corrections – that the random alloy structure of (Ca_2/3Ce_1/3)(Ti_1/3Mn_2/3)O₃ (CCTM2112) with Ce on the A-site is stable, as confirmed by X-ray crystallography measurements of a powder synthesized product with a similar bulk stoichiometry. We also predict that it offers an ideal reduction enthalpy for STCH, as corroborated by stagnation flow reactor experiments, demonstrating that CCTM2112 outperforms SLMA and BCM under similar experimental conditions. Additionally, DFT calculations reveal, and X-ray absorption spectra verify that, unlike other Ce-containing oxide perovskites for STCH like BCM, A-site Ce⁴⁺ is the dominant acceptor of the electrons left behind by neutral oxygen vacancy formation, which theory suggests happens even when Ce⁴⁺ does not neighbor the vacancy and derives from the metallic electronic structure of CCTM2112 and its associated high density of energetically accessible (and thus easily reducible) unoccupied Ce 4f states. Finally, I will explain intuitively the local-composition dependence of the reduction enthalpy (i.e., the oxygen vacancy formation energy) based on crystal bond dissociation energies and the electrostatic interactions between oxygen-vacancy-generated charge carriers.